Full color organic electroluminescent (EL), also known as organic light-emitting devices (OLED), have been demonstrated recently as a new type of flat panel display. OLED devices are attractive because of their low driving voltage, high luminance, wide-angle viewing and capability for full color flat emission displays. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic EL medium sandwiched between these electrodes to support charge recombination that yields emission of light. An example of an organic EL device is described in commonly assigned U.S. Pat. No. 4,356,429. Other examples have been described by Tang et al. in U.S. Pat. Nos. 4,769,292 and 4,885,211. In order to construct a pixilated display device that is useful, for example as a television, computer monitor, cell phone display or digital camera display, individual organic EL elements can be arranged as an array of pixels in a matrix pattern. This matrix of pixels can be electrically driven using either a simple passive matrix or an active matrix driving scheme. In a passive matrix, the organic EL layers are sandwiched between two sets of orthogonal electrodes arranged in rows and columns. An example of a passive matrix-driven organic EL device is disclosed in commonly assigned U.S. Pat. No. 5,276,380. In an active matrix configuration, each pixel is driven by multiple circuit elements such as transistors, capacitors, and signal lines. Examples of such active matrix organic EL devices are provided in U.S. Pat. Nos. 5,550,066 (commonly assigned), 6,281,634, and 6,456,013.
One way of improving the efficiency of an OLED device is the use of a microcavity structure. A reflector and a semitransparent reflector function, with the layers between them, form a microcavity, which can be adjusted in thickness and refractive index to resonate at a desired wavelength. Examples of microcavity structures are shown in U.S. Pat. No. 6,406,801, U.S. patent application Publication No. 5,780,174, and JP 11-288786.
Destructive light interference can result from microcavity effects within an OLED device and can cause color distortion when the OLED is viewed from oblique angles. Microcavity devices are characteristically directional; the emission intensity falls rapidly with viewing angle, e.g. N. Takada, T. Tsutsui, and S. Saito, Appl. Phys. Lett. 63 (15) 2032 (1993), “Control of emission characteristics in organic thin film electroluminescent diodes using an optical microcavity structure”.